1. Field of the Invention
The invention relates to a variable optical attenuator of light for use in optical communications.
2. Prior Art
Presently, with the wavelength multiplexing transmission systems in optical communications, there is known such a system as shown in FIG. 1. This system has a plurality of optical amplifiers 31 arranged on a transmission line 30 of wavelength multiplexed light at relaying positions so that the optical amplifiers 31 amplify the wavelength multiplexed transmission light. Such transmission accompanied with the amplification of wavelength multiplexed transmission light by a plurality of optical amplifiers 31 allows long-distance wavelength multiplexing transmission.
In the wavelength multiplexing transmission system shown in FIG. 1, a function of amplifying multi-wavelength light collectively is requires of each optical amplifier 31. For the wavelength multiplexing transmission system to improve in transmission quality, it is also required that the plurality of optical amplifiers 31 each perform optical amplification without a great difference in power among the individual wavelengths of the transmission light amplified. To reduce this differences in power among the wavelengths of the transmission light, it has been suggested that a variable optical attenuator having the function of uniformizing the multi-wavelength light to a desired power collectively be arranged, for example, in each of the optical amplifiers 31.
Among the requirements for this type of variable optical attenuator are: {circle around (1)} a constant optical attenuation at each wavelength of the multi-wavelength light, or equivalently, no variation or smaller variations in optical attenuation with respect to changes in wavelength; {circle around (2)} an attainable optical attenuation of −30 dB or higher; {circle around (3)} resistance to high optical input power; and {circle around (4)} a compact size.